With respect to a steel pipeline for conveying liquid hydrocarbons, it is a common requirement to have to extend the pipeline or to replace a defective section of it. These two operations are hereafter generically referred to herein as adding a section of pipe to the line.
In the particular case of replacing a section of defective pipe, one firstly drains liquid hydrocarbons from the section of the line to be worked on and then cuts or severs the line at the two ends of the section. Cutting can be carried out without danger using an explosion-proof cutting tool, specifically designed to operate in this environment.
In both cases, one is now at the point where the line is open-ended, hydrocarbon vapours are present in the line bore, and air may enter the bore.
To add a section of pipe will normally entail buttwelding.
There is therefore a need to seal the open end(s) of the line to isolate the hydrocarbon vapours and permit the addition of the new pipe section to go forward.
In commercial practice in the past, this has usually been accomplished by stuffing the open pipe end with a plug of particulate aquagel or similar material. After the new pipe section has been added, the plug is disintegrated by pumping it down the line using liquid hydrocarbon. The powder-like aquagel becomes dispersed and entrained in the flow. The aquagel particles are minute and do not damage pumps in the line. The concentration of particles in the liquid is so small that they essentially become inconsequential.
However the aquagel plugs have proven to be unreliable with respect to sealing off the hydrocarbon vapours. The plug can slump and/or move, allowing vapours to escape past it. The gas combined with the air trapped in the new section can result in an explosive mixture, which can be ignited by the welding process. In addition, the installation of the aquagel plug is a time-consuming process.
It is the objective of this invention to provide a new form of disintegratable plug for use in this service.
Turning now to prior art of interest known to applicant, reference is made to my U.S. Pat. No. 4,634,040. This patent teaches internally insulating a pipeline wall, that is to undergo a fillet weld, with a ring formed of fibrous material held together with a wax binder. The ring functions to prevent heat being conveyed away at too rapid a rate into liquid present in the pipe bore; such rapid heat loss can lead to development of a brittle weld. Upon completion of welding, the ring is disintegrated by flowing oil past it to dissolve the wax to leave the fibers, which can be pumped down the line without difficulty and, like the gel, become inconsequential.